To the Editor: In a letter in the December 1 issue, Gahl and Rizzo conclude that “cystinuric breast duct epithelium apparently does not express the dibasic amino acid-transport defect characteristic of renal tubular and intestinal cells.”1 The data presented are not sufficient to support such a conclusion. There are several fundamental differences between mammary epithelia and renal tubular or intestinal epithelia. Mammary transport systems move substrates from serosa to mucosa in order to secrete milk. Renal tubular cells may transport substrate in either direction, depending on the substrate or the location in the tubule. The dibasic-amino acid transport system moves.
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